Now up from Geochem J (vol. 60 #2):
Matsumoto, Y. Tachibana, S. Dehydration of MgHPO4·3H2O (newberyite) under low pressure conditions and its implications for the surface thermal history of Ruygu and Bennu p7 .GJ26002
Time for some more experimental (laboratory) astronomy. Phosphates (salts based on the PO4 anion) are present on Ryugu… and common on Bennu. (We have also found small amounts in meteorites- pieces of other asteroids.) These minerals are water (hence, the “·3H2O” in the formula), form in water, and can be used as water sources. But what sort of process(es) formed (and modified) the phosphates on asteroids Ryugu and Bennu?
Matsumoto and Tachibana try to find out. They ran numerous experiments in the lab, trying to take phosphates in their new, fully-hydrated form, and dehydrate them using various conditions that might exist on asteroids. Low pressure, high temperature, etc. may have occurred in the history of these and other bodies- did they? One of the results is that MgHPO4 is initially vulnerable to dehydration… but not completely. Getting that last water out of the mineral is quite difficult. Another result is that, based on these dehydration processes, Ryugu and Bennu did not get that last water out: at no point did the two bodies ever get that hot. It appears that, in the dynamics of the Solar System, neither Ryugu nor Bennu have been shuffled around into orbits that come inside Venus.